In recent years, natural energies, such as biomass energy, nuclear energy, wind-force energy, and solar energy have been currently studied as alternative energies for fossil fuel energies, such as oil and natural gas. Among these alternative energies for fossil fuel energies, the utilization of solar energy is considered as being prosperous as the most stable and voluminous natural energy. However, although the solar energy is a remarkably dominant alternative energy, the following points may be considered to become problems from the viewpoints of utilization of it. (1) The energy density of the solar energy is low, and (2) it is difficult to storage and transport the solar energy.
For the above problems of the solar energy, in order to solve the problem of the low energy density of the solar energy, a method of collecting the solar energy by a large-scaled reflecting apparatus has been proposed. As one of such a solar thermal power generation system, for example, PTL 1 describes a tower type solar thermal power generation system. This system includes a plurality of reflective mirrors arranged in the form of an approximately circle or an approximately fun and a tower installed in the central portion, and the system is configured to collect light by concentrating solar light rays via the reflective mirror into a heat collecting section disposed in the tower and to generate electric power by utilizing the heat of the collected light.
Here, as with the tower type solar thermal power generation system, in solar thermal power generation systems in which the distance from reflective mirrors to a heat collecting section is so long from some tens of meters to some hundreds of meters, light collection efficiency has not yet been sufficient, and further improvement is still required for the light collection efficiency. Hereinafter, this point will be described in detail.
Solar light rays are not perfect parallel light rays and are light rays with an inclination within an angle corresponding to a view angle of 0.52 to 0.54. In the case where the distance from a reflective mirror to a heat collecting section is as short as several meters, this view angle of the solar light rays may be almost disregarded. However, as with the tower type solar thermal power generation system, in the case where the distance from a reflective mirror to a heat collecting section becomes long, if the reflective mirror is a flat mirror, there are the following problems. When solar light rays are reflected on the flat mirror, among the reflected light rays of the solar light rays, light rays of a light-ray component corresponding to the view angle diffuse in proportion to the light collecting distance. Accordingly, the limited light receiving area of the heat collecting section cannot receive all of the reflected light rays, which results in that the light collection efficiency lowers.
In order to solve the above problems, PTL 1 describes the structure that as shown in FIG. 6, a pseudo concave mirror is configured by combining a plurality of flat mirrors. However, such a pseudo concave mirror is not sufficient from the viewpoint of light collection efficiency.
Further, in order to acquire a concave mirror from curves surfaces without combining the flat mirrors, a complicated manufacture process is needed. Accordingly, it has been difficult to produce such a concave mirror in a simple manner at low cost. In particular, in the case of using concave mirrors for the tower type solar thermal power generation system, it is required to change the curvature of a concave mirror in accordance with the distance from the heat collecting section to the concave mirror. It may be more difficult to produce such concave mirrors with various curvatures at low cost. As a result, the solar thermal power generation system including a plurality of such concave mirrors with various curvatures naturally becomes high cost.
Then, as with the tower type solar thermal power generation system, in a solar thermal power generation system in which the distance from a reflective mirror to a heat collecting section becomes so long from some tens of meters to some hundreds of meters, it is requested to realize a solar light collecting mirror which can obtain high light collection efficiency, can be produced easily at low cost, and enables to obtain concave mirrors with various curvatures easily.